1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus, and particularly to image forming apparatuses, such as electrophotographic copying machines, printers, and facsimile apparatuses, and fixing apparatuses usable therein.
2. Related Background Art
In recent years, coloring has been advancing in image forming apparatuses, such as copying machines and printers. In connection with an electrophotographic color image forming apparatus, a so-called in-line image forming apparatus has been proposed in which an array of photosensitive drums are arranged corresponding to respective colors, and toner images of respective colors formed on the respective photosensitive drums are sequentially superimposed on a transferring medium such that a color image can be formed.
As a fixing apparatus to be used in such a color image forming apparatus, a thermal roller fixing apparatus with a fixing member having an elastic layer is well known. In such a thermal roller fixing system using the elastic layer, there is posed a problem that thermal capacity of the thermal roller itself tends to be large, and it likely takes a long time (warm-up time) to heat the fixing roller up to temperature suitable for fixation of a toner image. This problem causes a user to wait for start-up of the apparatus for an unnecessarily long time, and is also undesirable in the light of consumption of electric power. Further, cost of the fixing apparatus is liable to increase.
As a fixing apparatus capable of achieving a short warm-up time, a fixing apparatus of a belt fixing type is well known. This type is often used in a monochromatic (black and white) printer. FIG. 14 schematically illustrates the structure of a model of such a belt fixing apparatus.
In FIG. 14, reference numeral 201 designates an entire structure of the belt fixing apparatus. Reference numeral 202 designates a fixing belt unit which is an assembly including a trough-shaped heater holder 207 having an approximately semicircular arcuate cross section, a fixing heater fixed to a lower surface of the heater holder 207 along its extension direction (a direction perpendicular to a sheet of FIG. 14), a thin fixing belt 203 of an endless belt configuration (a cylindrical shape) externally wound loosely around the heater holder 207 with the fixing heater 204, and so forth.
Reference numeral 205 designates an elastic pressure roller which is arranged with opposite ends of its metal core being freely rotatably supported by side plates of the fixing apparatus.
The fixing belt unit 202 is disposed above and parallel to the elastic pressure roller 205 with a side of the fixing heater 204 facing downward. And, the heater holder 207 is pressed downward with predetermined pressure created by a biasing unit (not shown) which acts on opposite ends of the heater holder 207. Accordingly, the lower surface of the fixing heater 204 is brought into pressure contact with the upper surface of the elastic pressure roller 205 against its elasticity with the fixing belt 203 being sandwiched therebetween. A fixing nip portion 206 with a predetermined width is thus formed.
The elastic pressure roller 205 is driven and rotated at a predetermined circumferential rate in a counterclockwise direction of an arrow by a driving mechanism (not shown). Due to the rotational driving of the elastic pressure roller 205, friction force between the elastic pressure roller 205 and the fixing belt 203 occurs at the fixing nip portion 206, and hence rotational force acts on the fixing belt 203. The fixing belt 203 is accordingly rotated around the outer surface of the heater holder 207 in a clockwise direction of an arrow at a circumferential speed approximately corresponding to the circumferential speed of the elastic pressure roller 205, while the inner surface of the fixing belt 203 is in close contact with and is slid on the lower surface of the fixing heater 204 at the fixing nip portion 206.
The fixing belt 203 is an endless belt of heat resisting resin with a thickness of about 50 microns, for example. A separating layer (fluorine coating resin, or the like) with a thickness of 10 microns is formed on the surface of the heat resisting resin. Further, no elastic layer is used in the fixing belt 203 to decrease its thermal capacity.
The fixing heater 204 is a ceramic substrate with a resistance heating body formed thereon. A temperature detecting unit 209 is disposed in contact with the fixing heater 204 such that temperature of the fixing heater 204 can be detected. Electric power supply to the fixing heater 204 is controlled by a control unit (not shown) such that its temperature can be controlled and reach a desired temperature.
Under a condition under which the elastic pressure roller 205 is driven and rotated, the fixing belt 203 is accordingly rotated, and the fixing heater 204 is heated up and adjusted to a predetermined temperature, a recording material P bearing unfixed toner images t is guided into the fixing nip portion 206 between the fixing belt 203 and the elastic pressure roller 205. An unfixed toner image bearing surface of the recording material P is brought into close contact with the outer surface of the fixing belt 203, and the recording material P is nipped at and conveyed through the fixing nip portion 206 simultaneously with the rotation of the fixing belt 203. During the nipped conveyance of the recording material P, heat of the fixing heater 204 is transmitted to the recording material P through the fixing belt 203, and the recording material P is subjected to pressure of the fixing nip portion 206. The unfixed toner image t is thus fixed on the recording material P as a permanent fixed image by those heat and pressure. Upon passing of the recording material P through the fixing nip portion 206, the recording material P is self-stripped from the surface of the fixing belt 203 by curvature, and discharged.
In the thus-constructed fixing apparatus 201, thermal capacity of the fixing belt 203 is made so small that the fixing nip portion 206 can be heated to temperature for enabling fixation of the toner image in a short time immediately after supply of electric power to the fixing heater 204.
However, when such a belt fixing apparatus 201 using the fixing belt 203 without the elastic layer is used as the fixing apparatus of the color image forming apparatus, the following situation occurs since no elastic layer is provided on the fixing belt 203 serving as a fixing member. The surface of the fixing belt 203 cannot follow unevenness of the surface of the recording material P, unevenness resulting from presence and absence of the toner layer, and unevenness of the toner layer itself, and hence a difference in heat transmitted from the fixing belt 203 appears between a concave portion and a convex portion on the recording material P. Sufficient heat is transmitted from the fixing belt 203 at the convex portion in close contact with the fixing belt 203, while only less heat is transmitted from the fixing belt 203 at the concave portion in less contact with the fixing belt 203 than at the convex portion. Thus, the toner layer reflects a difference in its melt condition due to the unevenness, and hence the fixed image is likely to be affected.
Particularly, in the color image forming system, toner images of plural colors are superimposed and mixed, so that its unevenness of the toner layer is larger than that in the monochromatic image forming system. Therefore, when no elastic layer is provided on the fixing belt 203, unevenness of gloss of the fixed image increases, and image quality is hence lowered. Further, in the event that the recording material P is an OHP sheet, when the fixed image is projected, light scattering occurs due to microscopic unevenness of the surface of the fixed image, and permeability is resultantly lowered.
Further, in the event that the fixing belt 203 is coated with silicone oil or the like such that sufficient heat can be fully transmitted to the fixing belt 203 without the elastic layer, the recording material P, and the uneven portion of the unfixed toner image t, the cost is likely to increase, and the fixed image and the recording material P are liable to be sticky due to the oil.
In such a situation, an inexpensive color on-demand fixing apparatus using a fixing belt with an elastic layer as the belt fixing apparatus has been proposed (see Japanese Patent Application Laid-Open No. H11-15303, for example).
FIG. 15 schematically illustrates the structure of the belt fixing apparatus using a fixing belt 203 with an elastic layer as the fixing member. In FIG. 15, members and portions common to those in FIG. 14 are designated by like reference numerals, and description thereof is omitted.
When this fixing apparatus is used, heat conductivity of a silicone rubber layer used as the elastic layer of the fixing belt 203 is small. Accordingly, temperature response of the fixing belt 203 is poor, and temperature of a sleeve following the temperature of the fixing heater 204 is largely delayed in response. Further, a difference in temperature between the fixing heater 204 and the fixing belt 203 is very large, say several tens degrees (° C.), even in a stationary state, and the temperature difference largely varies between idling rotation time and sheet passing time. Accordingly, it is very difficult to control the temperature of the fixing belt.
Therefore, a temperature controlling method as illustrated in FIG. 15 is proposed in place of the method using the fixing heater portion as in the apparatus illustrated in FIG. 14. In the temperature controlling method of FIG. 15, a temperature detecting unit 209 is provided on the surface or inner surface of the fixing belt 203 to detect the temperature of the fixing belt 203 itself, and the temperature of the fixing heater 204 is controlled by feedback control, such as PID control (Proportional-Integral-Differential), such that the temperature of the fixing belt can be adjusted. When such a construction is used, the temperature of the fixing belt 203 can be controlled more precisely.
This fixing apparatus, however, has the following disadvantages.
1) Heat conductivity of the silicone rubber layer used as the elastic layer of the fixing belt 203 is small, and many members are present in a location from the fixing heater 204 to the surface of the fixing belt. Accordingly, a so-called heat response, i.e., a time speed from the start of supply of electric power to the fixing heater 204 to rise of the temperature, is slow.
2) Location of the temperature detecting unit 209 for detecting the temperature of the fixing belt 203 is away from the fixing nip portion 206, and hence detection timing of the fixing nip portion is likely to be delayed.
Thus, dead time (time lag) is comparatively long for those two reasons. The feedback control represented by the PID control is accomplished by detection of variations in a control amount, and supply of an operation amount corresponding thereto. Therefore, it takes much time for the temperature of the fixing belt 203 to reach an appropriate temperature from the start of supply of electric power subsequent to the detection of variations in the control amount. As a result, overshoot and undershoot are likely to occur, and large hunting (temperature ripple) is likely to appear.
The above problems are especially outstanding (1) immediately after the start-up, and (2) at the time of start of sheet passing. As a method for coping with those problems, it is known that the following methods are very effective.
(1) In a first method, a first electric power level for speedily starting up the temperature of the fixing apparatus and a second electric power level for stabilizing the temperature of the fixing apparatus are prepared at the time of the start-up of the fixing apparatus, and operation is advanced to feedback control after a necessary electric power value set by considering a heat storage condition of the fixing apparatus is supplied for a predetermined time of period.
(2) In a second method, PID control is not executed for a predetermined time of period in synchronization with the rush-in timing of the recording material P at the time of start of the sheet passing, and when electric power to be supplied to the fixing heater 16 is corrected to a predetermined value and then input, the electric power is corrected to an approximately necessary electric power value set by considering the thermal characteristic of the recording material P and the heat storage condition of the fixing apparatus.
When the above-discussed control is executed, it is necessary that the predetermined electric power value of the second power level at the start-up time, and the predetermined power value to be corrected at the time of starting the sheet passing are approximately equal to the electric power value necessary for stabilization of the temperature of the fixing apparatus at a target temperature at the start-up time, and the electric power value needed at the time of sheet passing, respectively. In the event that the predetermined power value is greatly different from the necessary electric power value, temperature is likely to be remote from the target temperature, and hence the temperature ripple is liable to increase.
In the above fixing apparatus, wave-number control or phase control is used as the output control of electric power, in which the electric power is controlled in a manner that the output is determined by a percentage (%) of the maximum supply power (full power), but not in a manner that the output is determined by a value of watt. In other words, it is necessary to control the electric power value needed for control of the temperature by using the percentage (%) of the maximum supply power.
On the other hand, the maximum supply power fluctuates due to variations in input voltage into the fixing heater 204 and resistance value of the fixing heater 204. Table 1 shows variations in voltage, resistance and electric power in this fixing apparatus to be used in a region of 120 V. In this table, the range of the input voltage is 85% to 110% of a rated voltage, and variation in the resistance is ±7%.
TABLE 1Variations in voltage, resistance and electric power in thisfixing apparatus to be used in a region of 120 VLower limitUpper limit120-V regionof powerTypicalof powerVoltage102 V120 V132 VResistance13.91 Ω13 Ω12.09 ΩElectric747 W1,107 W1,441 Wpower
Here, the variation in the maximum supply power to the fixing apparatus 204 ranges from 747 W to 1,441 W, i.e., the maximum value is about twice the minimum value. When the above-discussed control (1) or (2) is executed, a center value of the maximum supply power is 1,107 W, and hence power of 332 W is output where 30% thereof is output as a predetermined electric power. In contrast, the predetermined power is 224 W where the lower power limit of 747 W is used, and the predetermined power is 432 W where the upper power limit of 1,441 W is used. Accordingly, under a condition under which supply of the predetermined power of 332 W is optimum, for example, a large temperature ripple is liable to occur at the time of input of the predetermined power due to the variation in the predetermined electric power accompanying the variation in the above maximum supply power.
Specifically, the temperature ripple becomes about 12° C. at upper and lower limits of the maximum supply power. In an in-line electrophotographic color image forming apparatus used in a test, gloss of an output printed matter fluctuates about seven (7) in monochrome, and the gloss fluctuates about eleven (11) in secondary color. Quality of the image is thus lowered (see Table 2). Further, poor fixation, such as hot offset and degradation of fixing characteristic, is likely to appear accompanying a large fluctuation in temperature, depending on a recording material and an image pattern.
TABLE 2Variation in gloss at upper and lower limits ofthe maximum supply power in a region of 120 VPrior artGloss averageVariation widthMonochrome(M/S = 0.55)Yabout 13about 7Mabout 13about 7Cabout 12about 7Kabout 9about 6Secondary color(M/S = 1.2)Rabout 19about 11Gabout 18about 11Babout 18about 11
In the event that the maximum supply power is large, overshoot at the start-up time becomes excessively large. Accordingly, if use is repeated, operations at higher temperatures are repetitively performed, and hence life of the fixing apparatus is liable to be short.
Further, excessive overshoot causes a large loss even in the light of consumption of electric power, and electric power is likely to be unnecessarily consumed wastefully.
Here, the resistance of the fixing heater 204 in the 120-V region is assumed to be 13.0 Ω. In a region where the rated voltage is 127 V, however, when a fixing heater having the same resistance is used, it is necessary to consider that the maximum supply power to the fixing heater 204 can be up to 1,614 W if variation up to 110% and variation in the resistance value are taken into consideration.
Further, when use in a 100-V region is considered, it is necessary to consider that the maximum supply power to the fixing heater 204 can be up to 519 W if variation in the rated voltage up to 85% of 100 V and variation in the resistance value are taken into consideration.
To sum up, the variation in the maximum supply power to the fixing heater 204 ranges from 519 W to 1,614 W, which is about three times larger than 519 W.
In such a case, control of the temperature becomes more unstable for similar reasons. Accordingly, quality of the image is further lowered due to variation in gloss, and worse fixation, such as hot offset and degradation of fixing characteristic, is likely to occur depending on the recording material and the image pattern. Further, when the maximum supply power is large, overshoot at the start-up time becomes larger. Accordingly, if use is repeated, operations at still higher temperatures are repetitively performed, and hence life of the fixing apparatus is liable to be further shortened. In addition, consumption of electric power further increases.
To cope with the above problem, there has been proposed a method in which the resistance value of the fixing heater 204 is selectively set in conformity with the rated voltage in each region. In this case, however, costs of the fixing heater and management increase. Further, if the apparatus is used in a different region other than its destination region, or if the apparatus is erroneously destined to a different region, the above problem occurs, and accordingly there occur a fear that the user becomes unsatisfied, and a fear that expenditure for service resultantly increases.
Here, even when the heater is selectively set for each destination as discussed above, the problem in connection with upper and lower limits of the maximum supply power in each region itself still remains. In other words, a region where an electric power source is considerably unstable exists among regions where the fixing apparatus is used, and a case where a range of input electric power greatly differs from the rated voltage occurs. Also in such a case, similar problem arises consequently.